Re: Hush Kits

From:         Don Stokes <>
Organization: Victoria University of Wellington
Date:         13 May 96 02:08:44 
References:   1
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  or MIME structure writes:
>I understand that one of the main reasons for fitting such kits to older
>aircraft is to increase fuel efficency, reduce noise and apply more modern
>drag reducing measures to ageing designs.  I may however be wrong.  :)

Yup, you're wrong.  8-)

Hushkits are installed *soley* to cut noise to meet Stage III requirements.
They actually reduce the aircraft's efficiency (by a small %age) due to
reduced thrust and a little more drag.

>What would be involved in fitting such a kit?  New fan blades,  possibly
>wing-lets or am I way off the mark?

The hushkits on Air NZ's 737-200s are basically an extra bypass section
bolted onto the tailpipe, replacing the thrust reversal buckets.  (The
hushkit either has its own reversal buckets, or they re-install the old
buckets on the back of the hushkit -- I'm not sure, but they look very
similar.)  The kit mixes some of the surrounding air with the engine core
flow, so that the aggregate speed of the flow hitting still air outside
the tailpipe is slower than it would be

                Bypass           Turbine      Hushkit
                  \                /      __    /
          __-------\--------------/--____   ---/____
            | __-----------------/-____  ---        Hushkit flow
        Fan |    |||__----____|||||      ))))))))))))))) Bypass flow
           < ====   __    ____     >  >>>>>>>>>>>>>>>>>>>>>>> Core flow
            | __ |||  ----    |||||____  ))))))))))))))) Bypass flow
          __|   --/--------\-------  ____---    ____Hushkit flow
            -----/----------\--------     __----
                /            \
           Compressor      Combustion chamber

Now, I'm not sure what's actually inside the huskit cowling; I don't
believe there is any kind of fan there, but I'm not sure if the mixing
with the core & bypass flows is achieved by simply guiding the surounding
air into those flows or of there is a system of vanes inside the kit.

BTW: Concorde has variable exhause that performs a similar trick on
takeoff & landing -- the leading edge of the variable nozzle separates
from the tailpipe to gather surounding air and mixes it with the core
flow (the pure turbojets on Concorde have no bypass section), reducing
the noise from extremely loud to just very very very loud.  (The nozzle
also performs thrust reversal on landing -- it's a lot like the thrust
reversal buckets on the 737, but smaller and fully variable, acting as
an expansion chamber for supersonic flight.)

Don Stokes, Network Manager, Victoria University of Wellington, New Zealand. +64 4 495-5052 Fax+64 4 471-5386